Ultrashort-pulse source with controllable multiple-wavelength output

ABSTRACT

A multiple-wavelength ultrashort-pulse laser system includes a laser generator producing ultrashort pulses at a fixed wavelength, and at least one and preferably a plurality of wavelength-conversion channels. Preferably, a fiber laser system is used for generating single-wavelength, ultrashort pulses. An optical split switch matrix directs the pulses from the laser generator into at least one of the wavelength conversion channels. An optical combining switch matrix is disposed downstream of the wavelength-conversion channels and combines outputs from separate wavelength-conversion channels into a single output channel. Preferably, waveguides formed in a ferroelectric substrate by titanium indiffusion (TI) and/or proton exchange (PE) form the wavelength-conversion channels and the splitting and combining matrices. Use of the waveguide allows efficient optical parametric generation to occur in the wavelength-conversion channels at pulse energies achievable with a mode-locked laser source. The multiple-wavelength laser system can replace a plurality of different, single-wavelength laser systems. One particular application for the system is a multi-photon microscope, where the ability to select the ultrashort-signal wavelength of the laser source accommodates any single fluorescent dye or several fluorescent dyes simultaneously. In its simplest form, the system can be used to convert the laser wavelength to a more favorable wavelength. For example, pulses generated at 1.55 μm by a mode-locked erbium fiber laser can be converted to 1.3 μm for use in optical coherence tomography or to 1.04-1.12 μm for amplification by a Yterbium amplifier, allowing amplification of pulses which can be used in a display, printing or machining system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an apparatus and method forgenerating ultrashort optical pulses at a plurality of opticalwavelengths, and, more particularly, to an apparatus and method usingoptical fibers and optical waveguides to produce and control suchoptical pulses. Ultrashort is here generally referred to as being withinthe time scale of approximately 10⁻¹⁵ seconds (femtoseconds) to 10⁻¹²seconds (picoseconds). The present invention further relates to a methodand apparatus for optical imaging using ultrashort optical pulsessimultaneously emitted at a plurality of optical wavelengths.

2. Description of the Related Art

A variety of laser systems for producing ultrashort optical pulses isknown in the prior art. From a practical point of view, these systemscan be generally grouped into two main categories: solid-state lasersystems, which are based on the use of volume laser gain media, andfiber laser systems, which are based on waveguiding fiber-opticcomponents. Due to their intrinsic structure, fiber lasers have a numberof basic properties which make them significantly more suitable forwidespread practical use. As is well known in the prior art, fiberlasers are compact, can be diode pumped, and are robust and reliable.For a number of reasons, at present, the most mature technology suitablefor ultrashort-pulse fiber laser systems is based on Er-doped fiberproviding output pulses having a wavelength of approximately 1.55 μm.First, Er-doped fibers are among the best developed of-therare-earth-doped fibers. Diode lasers for pumping such fibers are alsowell advanced.

Significantly, the generation of ultrashort pulses requiresdesign-control of the dispersion in the laser cavity. This can beaccomplished in a compact, all-fiber cavity only at wavelengths above1.3 μm, where the dispersion of the optical fiber can be tailored to beeither of positive or negative sign. However, a variety of practicalapplications for ultrashort pulses require other wavelengths ofoperation, for example, either at shorter or longer wavelengths. Atthose wavelengths, femtosecond-pulse fiber oscillators at present can bedesigned only by using bulky external components, such as sets of prismpairs, to control the in-cavity dispersion.

For many applications, the wavelength of the laser is critical. Forexample, for confocal microscopy used in cellular biology, specific dyesare attached to different parts of the cell and are used to observedifferent functions. Each of these dyes is excited to fluorescence by arespective spectrum of light. Thus, for confocal microscopy, a pluralityof different wavelength lasers are used for different dyes. Recently,ultrafast lasers with short pulses and high peak power have been used toexcite dyes at resonances which require two-photon excitation. That is,ultrafast lasers have been used to supply enough photon density at thefocus of a microscope to cause a non-linear optical effect, called thetwo-photon absorption effect. This effect is used to excite dyes at theenergy level which corresponds to half of the wavelength of each of thetwo original photons. However, the number of lasers available atdifferent wavelengths is limited; consequently there are only a few dyeswhich can be utilized at this time. Therefore, the field of two photonmicroscopy could benefit greatly from a laser capable of being widelytuned to the different wavelengths corresponding to a number ofdifferent dyes. The current accepted specifications for a laser forscanning two-photon microscopy are 10-30 mw average power, 100-200 fspulse width and 50-100 MHZ repetition rate.

The general and well known method to extend the wavelength range of anyparticular laser system is to utilize nonlinear optical interactions,such as optical harmonic generation, sum or difference frequencygeneration and optical parametric gain.

Harmonic generation is suitable only for converting an optical signal toa higher optical frequency (shorter wavelength) and it cannot providetunable or multiple-wavelength output. Sum-frequency anddifference-frequency generation allows conversion of a signal to bothhigher and lower optical frequencies and allows wavelength tunability,but requires at least two well synchronized optical sources at twodifferent optical frequencies. Therefore, each of these interactionsalone cannot provide multiple-wavelength or wavelength-tunable outputfrom one, single-wavelength signal source.

Optical parametric interaction is suitable for providing tunable ormultiple-wavelength conversion using one, single-wavelength opticalsignal source. Furthermore, while optical parametric conversion allowsconversion of an optical signal only to a lower optical frequency(longer wavelength), by combining parametric interaction with at leastone of the above described interactions, any optical frequency above orbelow the signal-source frequency can be obtained.

The general drawback of parametric optical frequency conversion is that,in order to achieve high parametric gain sufficient to amplifyspontaneous quantum-fluctuation noise from microscopic to macroscopiclevels and, consequently, to achieve efficient signal-energy conversion,high peak-powers and high pulse-energies are required. It is well knownfrom the prior art that the required energies are well above theenergies that can be generated directly from a typical mode-locked,ultrashort-pulse laser oscillator. The best demonstrated result known todate is an optical parametric generation (OPG) threshold at ˜50 nJ, andefficient OPG conversion of ˜40% at approximately 100 nJ achieved inbulk periodically-poled lithium-niobate crystals, as reported byGalvanauskas et al. in "Fiber-laser-based femtosecond parametricgenerator in bulk periodically poled LiNbO₃ "; Optics Letters, Vol. 22,No. 2; January, 1997. In comparison, typical femtosecond mode-lockedpulse energies from a fiber laser are in the range of 10 pJ to 10 nJ (asdescribed by Fermann et al. in "Environmentally stable Kerr-typemode-locked erbium fiber laser producing 360-fs pulses"; Optics Letters;Vol. 19, No. 1; January, 1997, and by Fermann et al. in "Generation of10 nJ picosecond pulses from a modelocked fibre laser"; ElectronicsLetters, Vol. 31, No. 3; February, 1995) and those from a solid-statelaser are in the range of up to ˜30 nJ (as described by Pelouch et al.in "Ti:sapphire-pumped, high-repetition-rate femtosecond opticalparametric oscillator"; Optics Letters, Vol. 17, No. 15; August, 1992).

It is known from the prior art that efficient optical parametricwavelength conversion can be achieved with unamplified or amplifiedmode-locked laser pulses by arranging a nonlinear crystal in a separateoptical cavity in a manner that ensures that pump pulses and signalpulses pass the parametric gain medium synchronously, as seen, forexample in the above-referenced article by Pelouch et al. Since, in thiscase, parametric interaction occurs repetitively, the low, single-passparametric gain and, consequently, low pulse energies of mode-lockedoscillators are sufficient to achieve efficient conversion. Thesignificant practical drawback of this approach is that such a schemerequires two precisely length-matched optical cavities; one for amode-locked oscillator and another a for synchronously-pumped opticalparametric oscillator (OPO). Consequently, such OPO systems are complex,large, and intrinsically very sensitive to the environmental conditions(non-robust). Furthermore, wavelength tuning of such a system requiresmechanical movement of the tuning elements such as rotation ortranslation of a nonlinear crystal, rotation of cavity mirrors, etc.,which is incompatible with fast wavelength tuning or switching.Therefore, OPOs can not serve as practical ultrashort-pulse sources forproducing multiple-wavelength pulses directly with mode-lockedoscillator output.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a method andapparatus for generating ultrashort optical pulses at a variable oradjustable optical wavelength from a single source which providesultrashort optical pulses at a fixed optical wavelength.

It is a further object of the present invention to provide a method andapparatus for generating ultrashort optical pulses at a plurality ofoptical wavelengths using a single source which provides ultrashortpulses at a fixed optical wavelength.

Another object of the present invention is to provide fast control ofthe output of a laser system in order to select between a plurality ofwavelength conversion channels.

Still another object of the present invention is to provide a pluralityof wavelengths at the single output of a laser system by combiningoutputs from separate wavelength-conversion channels into a singleoutput beam.

Yet another object of the present invention is to enable efficientmultiple-wavelength or adjustable-wavelength operation at relatively lowpulse energies and powers which are compatible with existingultrashort-pulse laser oscillators. An additional object of the presentinvention is to implement such a system using components which arerobust, compact and well-suited for large-volume fabrication in order toprovide a compact, robust, easily manufacturable and cost-effectiveapparatus.

It is a further object of the present invention to implement suchmultiple-wavelength laser systems in optical imaging systems, where theability to select from a plurality of optical-signal wavelengths or tosimultaneously use a plurality of optical-signal wavelengths isessential to extend imaging capabilities.

The aforesaid objects are achieved individually and in combination, andit is not intended that the present invention be construed as requiringtwo or more of the objects to be combined unless expressly required bythe claims attached hereto.

In accordance with the present invention, these objects are achieved ina system having a first part comprising a laser system for producingultrashort pulses at a fixed wavelength, and a second part comprising atleast one and preferably a plurality of wavelength-conversion channels.A wavelength-controlling element (or elements) is disposed between thelaser generator and the wavelength-conversion channels, which element(s)directs the pulses from the laser generator into at least one of thewavelength conversion channels. Another component or plurality ofcomponents is disposed downstream of the wavelength-conversion channelsand serves to combine outputs from separate wavelength-conversionchannels into a single output channel.

According to the present invention, novel optical waveguide devices areused for the wavelength-conversion channels, wavelength-control andbeam-control elements. Preferably, a fiber laser system is used forgenerating single-wavelength, ultrashort pulses.

The multiple-wavelength laser system of the present inventionadvantageously replaces a plurality of different, single-wavelengthlaser systems. One particular application for the system is amulti-photon microscope, where the ability to select theultrashort-signal wavelength of the laser source accommodates any singlefluorescent dye or several fluorescent dyes simultaneously.

Another application for the present invention is in systems that requireultrashort optical pulses at wavelengths that are different from thewavelength of the pulse-generating laser. For example, the system of thepresent invention can shift the ultrashort pulse wavelength toapproximately 1.3 μm for optical coherence tomography (OCT), wheretissues are most transparent. Similarly, the system of the presentinvention is capable of shifting the wavelength of the ultrashortoptical pulses into a range of wavelengths (1.04 to 1.12 μm) which canbe amplified by Yterbium amplifiers to produce very high powerultrashort pulses for applications such as machining, printing anddisplays.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

All of the above-referenced articles are incorporated herein byreference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an ultrashort-pulse laser sourceaccording to the present invention.

FIG. 2 is a diagrammatic view of a preferred waveguide structure for thewavelength conversion channels of the present invention.

FIG. 3 is a graph illustrating the theoretical optical parametricgeneration (OPG) threshold-energy dependence on the pump pulse(in-waveguide) duration for bulk and waveguide structures inperiodically-poled lithium niobate (PPLN).

FIG. 4 is a graph illustrating the measured optical parametricgeneration (OPG) conversion efficiency as a function of pump energyaccording to the present invention.

FIG. 5 is a graph illustrating the measured signal and idler wavelengthsversus pump wavelength at 100° C.

FIG. 6 is a diagrammatic view of multiple-wavelength output using asingle waveguide in accordance with the present invention.

FIG. 7 is a diagrammatic view of a multiple-wavelength, ultrashort-pulsegenerating system according to the present invention.

FIGS. 8-10 are diagrammatic views of an optical combining switch matrix,formed in a surface of a substrate, for switching pulses in one or bothof two waveguides into an output waveguide.

FIG. 11 is a diagrammatic view of an optical combining switch matrix(OCSM) capable of combining ultrashort optical pulses traveling in threewavelength conversion channel waveguides into a single output waveguide.

FIG. 12 is a diagrammatic view of an optical split switch matrix (OSSM)for selectively distributing ultrashort pulses into three wavelengthconversion channels from one, single-wavelength pulse source.

FIG. 13 is a diagrammatic view of an optical split switch matrix (OSSM)for selectively distributing ultrashort pulses into three wavelengthconversion channels using an acousto-optic device.

FIG. 14 is a diagrammatic view of an optical split switch matrix (OSSM)for selectively distributing ultrashort pulses into three wavelengthconversion channels using an electro-optic device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a top-level diagram illustrating a system for providingultrashort pulses with an adjustable or variable optical wavelength or aplurality of wavelengths according to the present invention. The systemincludes an ultrashort pulse laser (UPL) 10 for producing ultrashortoptical pulses at a fixed wavelength, and at least onewavelength-conversion channel (WCC) 12₁ -12_(n),

UPL 10 is preferably a mode-locked fiber oscillator which can providepicosecond or femtosecond optical pulses with typical pulse energiesbetween 10 pJ (10×10⁻¹² J) to 10 nJ (10×10⁻⁹ J) and typical averagepowers between 0.1 mw to 100 mw. The mode-locked fiber oscillator canhave any of a variety of possible designs, such as those described inthe above-referenced articles by Fermann et al. It is preferable, forthe reasons outlined above, that the fiber oscillator have an all-fibercavity without any non-fiber dispersion-controlling elements.Consequently, the preferable operation wavelength is 1.55 μm.

One important feature of the embodiment illustrated in FIG. 1 is thatthe wavelength conversion in the WCCs is obtained in an opticalwaveguide. As explained above, use of nonlinear conversion in the volumeof currently known nonlinear materials does not allow optical parametricgeneration to be achieved using unamplified output from a mode-lockedfiber laser or, in general, from any other existing mode-lockedultrashort-pulse laser. It has been experimentally demonstrated for thefirst time that, by using optical parametric generation in speciallydesigned waveguides in periodically-poled lithium niobate (LiNbO₃), theOPG threshold can be lowered into the energy range accessible withultrashort-pulse oscillators.

An essential difference between parametric generation in a bulk crystaland in an optical waveguide is that the latter allows the optical beamto be confined to a small cross-sectional area and allows the opticalbeam to propagate along the whole waveguide length without diffractionspreading. In contrast, propagation of a free-space beam in a volume ofan optical crystal results in diffraction spreading. Consequently, asignificantly higher optical intensity over a long propagation length inan optical waveguide results in significantly higher optical parametricgain compared to bulk crystal for the same optical pump power.

Furthermore, maximum interaction length between two or more ultrashortpulses is limited due to different group-velocities at different opticalwavelengths. This maximum, walk-off limited length ι_(walk-off) isdetermined by the duration of the pulse Δτ and the group-velocitymismatch (GVM) parameter υ_(GVM) of an optical material: ι_(walk-off)=Δτ/υ_(GVM).

Quantitatively, the advantage of OPG in an optical waveguide compared tothe confocally focussed beam in the bulk of the same nonlinear material(at the degeneracy) can be expressed by the following formula: ##EQU1##

Here, P_(th).conf. and P_(th).waveg. are threshold peak-powers for pumppulses in a bulk crystal and in a waveguide, respectively, λ and n aresignal wavelength and refractive index at the degeneracy point, andA_(waveg). is the waveguide cross-sectional area. Higher thresholdpeak-power requires higher pump pulse energies. Therefore, the advantageof using an optical waveguide compared to a bulk crystal is inverselyproportional to the pulse duration. Note that, for a bulk material, OPGthreshold is independent of the pulse duration.

As described above, the lowest OPG threshold in bulk crystal has beenachieved in a periodically-poled lithium niobate (PPLN). Therefore, thepreferable material for a parametric waveguide is PPLN, although otherperiodically-poled ferroelectric optical materials such as PP lithiumtantalate, PP KTP, etc. can also be advantageously used. The opticalwaveguides are preferably fabricated in a PPLN substrate using knowntitanium indiffusion (TI) or proton-exchange (PE) (or a combination oftitanium indiffusion and proton-exchange (TIPE)) techniques.

FIG. 2 illustrates a preferred waveguide structure for the WCCs of thepresent invention. The optical parametric generation (OPG) stage 14 ispreceded by a segmented mode-converter structure 16. The mode-converter16 can have a design similar to that described by Chou et al. in"Adiabatically tapered periodic segmentation of channel waveguides formode-size transformation and fundamental mode excitation"; OpticsLetters, Vol. 21, No. 11; June, 1996, incorporated herein by referencein its entirety. Use of the mode converter 16 is advantageous, since theOPG waveguide is single-mode at the longer, parametrical signalwavelength (in this particular embodiment, at ˜1.55 μm) but ismulti-mode at the shorter, pump wavelength (in this particularembodiment, at 780 nm). Therefore, it is difficult to excite a single,fundamental mode at the pump wavelength in such a waveguide by a directcoupling of a pump into the waveguide. Best performance, in terms ofthreshold, stability and conversion efficiency can be achieved when thepump is coupled into this mode-converter port 16 first, where it isconverted into a fundamental mode, and then launched into the OPGsection 14 in a fundamental transversal mode.

FIG. 3 illustrates the theoretical OPG threshold-energy dependence onthe pump pulse (in-waveguide) duration for bulk (dotted line) andwaveguide (solid line) structures in PPLN. The OPG threshold energy iscalculated using the above formula and the measured 50 nJ OPGenergy-threshold for bulk PPLN. The experimental energy-threshold valuemeasured for 2 ps long pump pulses is ˜340 pJ, as shown in FIG. 3 by abullet. The threshold level constitutes a reduction of approximatelytwo-orders of magnitude for this particular pulse duration and thusdemonstrates that OPG can be achieved with picosecond and subpicosecondpulse durations essentially in the energy range accessible withmode-locked lasers. One example of such a fiber oscillator is given inthe above-mentioned article by Fermann et al. (Electronics Letters, Vol.31, No. 3), providing 6-10 nJ pulses for 2-4 ps duration, which aresufficient to directly drive the waveguide OPG wavelength conversionchannels of the present invention.

As illustrated graphically in FIG. 4, efficient energy conversion can beachieved with this structure. Maximum conversion efficiencies of ˜25%have been reached for pump energies at approximately 4-5 times the OPGthreshold.

The converted optical wavelength can be adjusted by adjusting thetemperature of the waveguide (i.e., by controlling the temperature ofsubstrates in which the waveguide is formed) thus accessing a pluralityof optical wavelengths with a single waveguide. The OPG 14 is capable ofsimultaneously producing two different optical wavelengths, the shorterof which is called "signal" and the longer of which is called "idler".Therefore, a single WCC is suitable for generating two required opticalwavelengths by properly choosing the pump wavelength and theperiodic-poling period such as to satisfy energy-conservation andmomentum conservation laws for all three (pump, signal and idler)optical wavelengths. As an example, FIG. 5 illustrates the measuredsignal and idler wavelengths versus pump wavelength at temperature of100° C. and a quasi-phase-matched (QPM) grating period of 15 μm.

Furthermore, multiple-wavelengths can be accessed with a single chipcontaining a plurality of waveguides with different electrically-poledperiods, as shown in FIG. 1. Each predesigned wavelength can be accessedby translating the crystal in the transversal direction to select therequired waveguide.

In accordance with the present invention, each WCC optionally includesat least one harmonic generator HG 18 before the OPG stage 14 and atleast one harmonic generator HG 20 at the PG stage 14. Generally, thisallows generation of optical wavelengths shorter than the mode-lockedlaser wavelength. All waveguides can be manufactured on a single chipthus simplifying the system and eliminating additional in-waveguidecoupling losses. If a free-space pump is first coupled into a waveguideharmonic-generator which is a single-mode at this initial wavelength,e.g., at 1550 nm, then the wavelength-converted beam at a shorterwavelength, in general, will be obtained in a fundamental mode and canbe directly launched into the OPG stage (on the same chip). Modeconverters then may not be necessary.

An example of a configuration with a multiple-wavelength output using asingle waveguide as a wavelength conversion channel is shown in FIG. 6.For example, for two photon microscopy, a specific set ofultrashort-pulse wavelengths at ˜680 nm, ˜780 nm and ˜915 nm is highlydesirable. This can be accomplished by launching a ˜1550 nm fiber-laserinput into a waveguide, the first section of which constitutes asecond-harmonic generator 60, implemented through the correct PPLNperiod (which should be designed taking into account the exact geometryof the waveguide) and the temperature of the waveguide substrate. Thewaveguide can have the same width throughout all sections, provided thatthe 1.55 μm input is single-mode. Then, the generated second-harmonic isin the fundamental mode. The doubled output of the fiber laser at ˜780nm is further transmitted into the OPG section 62 of the waveguide tosimultaneously generate ˜1360 nm as the signal wavelength, and ˜1830 nmas the idler wavelength. The specified wavelengths can be obtainedthrough a certain PPLN period used for the OPG section, according thefactors described above. These two generated signal and idlerwavelengths can be separately doubled in further sections of thewaveguide to provide 680 nm and 915 nm wavelength pulses, respectively.The remaining 780 nm pump is transmitted together with these twowavelengths to the output to be used, for example, for two-photonmicroscopy. The final stages of the device, containing harmonicgenerators 64 and 66 for OPG output can be implemented on the samesubstrate, separately on a different substrate or substrates or evenusing bulk material.

A general embodiment of a multiple-wavelength, ultrashort-pulsegenerating system according to the present invention is shown in FIG. 7.The system for generating and controlling a multiple-wavelengthultrashort-pulse output includes an ultrashort pulse laser (UPL) 10 forproducing ultrashort optical pulses at a fixed wavelength, an optionalultrashort-pulse amplifier (UPA) 22 for increasing power and energy ofthe ultrashort pulses from UPL 10, an optical split switch matrix (OSSM)24 for distributing ultrashort pulses into a plurality ofwavelength-conversion channels, at least one and preferably a pluralityof wavelength-conversion channels (WCCs) 12_(l) to 12_(n), each of whichincludes a parametric-generation stage (PG) 14 and optional harmonicgeneration (HG) stages 18 and 20, and an optical combining switch matrixOCSM 26 at the output of the system to combine output ports of aplurality of WCCs to provide a single output beam (the OSSM and OCSM arenot necessary if only one WCC is present).

If the pulse energy directly produced by a mode-locked fiber oscillatoris insufficient for driving the waveguide-WCCs, the laser output can beamplified in an ultrashort-pulse amplifier UPA 22. Preferably, suchamplifier is a fiber amplifier. Very importantly, the low energyrequired to operate the waveguide-WCCs allows one to use a relativelysimple fiber amplifier design. Pulses in the 1-10 nJ range and highercan be obtained either directly or by using compact and simplechirped-pulse amplification schemes based on chirped fiber gratings orchirped-period poled lithium niobate compressors C-PPLN, as described byGalvanauskas et al. in "Use of Chirped-Period-Poled Lithium Niobate forChirped Pulse Amplification in Optics Fibers"; Ultrafast Optics '97,Monterey Calif.; August, 1997, incorporated herein by reference in itsentirety.

The optical combining switch matrix (OCSM) 26 is capable of selecting aparticular laser source from a plurality of WCCs. A conceptual plan viewof a basic OCSM formed in a surface of a ferroelectric (e.g., PPLN)substrate is illustrated in FIG. 8. The basic OCSM is capable ofswitching pulses of wavelength λ₁ from waveguide 30 and/or pulses ofwavelength λ₂ from waveguide 32 into a main trunk (output) waveguide 34.As explained above, the substrate is preferably made of a ferroelectricmaterial such as lithium niobate or lithium tantalate. The opticalwaveguides are fabricated using titanium indiffusion (TI) orproton-exchange (PE) or a combination of titanium indiffusion andproton-exchange (TIPE). The optical switches are fabricated by bringingcertain regions of the two optical waveguides sufficiently closetogether that the laser light can be switch from one waveguide toanother.

As shown in FIG. 9, in the absence of an external electric field,ultrashort pulses in waveguides 30 and 32 will not be switched to themain trunk waveguide 34 and will continue to propagate in waveguides 30and 32 to optically terminated ports. The application of specificelectrical voltages will cause the complete coupling of the ultrashortpulses in waveguide 30 and/or waveguide 32 into main trunk waveguide 34.For example, as shown in FIG. 10, the ultrashort light pulses inwaveguide 32 are coupled into the main trunk waveguide 34 by applicationof voltage V2 across the gap between the two waveguides. Opticaldirectional couplers 36 and 38, such as those described in "Introductionto Optical Electronics", Amnon Yariv, pp. 391-395, Holt, Rinehart andWinston, 1976, incorporated herein by reference in its entirety, can beused to apply the respective electric fields between waveguides 30 and32 and main trunk waveguide 34. The main trunk waveguide 34 ispreferably fabricated using only the TIPE process. This allows thevarious ultrashort pulses to propagate relatively efficiently throughthe main trunk waveguide 34 and hence will provide a common port for allthe WCCs.

FIG. 11 is a diagrammatic plan view of the OCSM 26 shown in FIG. 7. FIG.11 shows an OCSM capable of handling three WCCs, although the OCSM canbe designed to handle any number of WCCs in accordance with theprinciples illustrated in FIG. 11. The OCSM 26 comprises three opticaldirectional couplers 40, 42 and 44 fabricated in a ferroelectricmaterial. The main optical waveguides 48, 52 and 56 of three WCCs arefabricated using TI. The waveguide 48 of a first WCC propagating 500 nmultrashort pulses forms the center input waveguide with a refractiveindex of n1. In the off state (zero applied voltage), the 500 nmultrashort pulses continue to propagate in waveguide 48 (which becomethe center portion of output waveguide 46). By application of a voltageV1, the 500 nm ultrashort pulses are switched to a laser dump port 50and absorbed. Within the output waveguide 46, the 500 nm pulses tend topropagate primarily within the region having the n1 refractive index,thereby preserving a high degree of single mode operation.

A second WCC waveguide 52 providing 780 nm wavelength pulses is coupledto the output waveguide 46 by the second hybrid optical directionalcoupler 42. In the off state, the 780 nm ultrashort pulses are dumpedinto an optically terminated port 54. By application of an electricalvoltage V2, the 780 nm ultrashort pulses can be switched to the outputwaveguide 46 with the hybrid TIPE waveguide of refractive index n2. The780 nm pulses propagate primarily within the portion of the outputwaveguide having n1 and n2 indices of refraction, thereby preserving ahigh degree of single mode operation (i.e., the combined cross-sectionalarea of the n1 and n2 regions is consistent with single mode propagationfor 780 nm ultrashort pulses).

The OCSM 26 further comprises another hybrid optical directional coupler44 and an additional TIPE waveguide section of the output waveguide 46with refractive index n3, where n1>n2>n3. The role of this additionalTIPE waveguide section is to enable the propagation of the 980 nmwavelength pulses within the output waveguide 46. Specifically, the 980nm pulses propagate primarily across the n1, n2 and n3 regions, wherethe combined cross-sectional area of the n1, n2 and n3 regions isconsistent with single mode propagation for 980 nm ultrashort pulses).If zero voltage is applied to the third directional coupler 44, the 980nm ultrashort pulses propagating in waveguide 56 are dumped in thetermination port 58. By applying an electrical voltage of V3 to thehybrid optical directional coupler, the 980 nm wavelength pulses areguided in the common hybrid output port 46.

Although, for convenience, the output waveguide 46 is shown in FIG. 11as having separate regions of n1, n2 and n3 refractive indices, it willbe understood that the refractive index changes gradually over the widthof the output waveguide 46, i.e., there is no refractive index "step"between regions n1 and n2 and between regions n2 and n3. Further, itwill be understood that the n1, n2 and n3 regions are side-by-side inthe substrate, e.g., the two n2 regions need not be a single regionextending below and around the n1 region.

The optical directional couplers are preferably hybrid, because the TIPEwaveguide technology is used here. The use of TIPE waveguides assists inthe ability to combine all three wavelength sources to exit from thesubstrate through a common port and all wavelengths can remain inquasi-single mode operation. As can be seen, significant complication isadded to the device so that the multiple wavelengths can propagate downa single waveguide and still be single mode. If the wavelengths areclose enough together, a single waveguide will be single mode for each.

The OCSM 26 shown in FIG. 11 can be extended to combine any number ofWCCs limited only by the size of the substrate material. With theavailability of 4" lithium niobate wafer material, it is possible tocombine up to ten different WCCs. The design of the various TIPEwaveguide sections is more critical with greater numbers of WCCs.

The above description relates to combining of signal pulses received ineach of the WCCs. The same principle can be extended to switching of theidler signal in each of the WCCs as well.

It should be noted that, as an alternative to the above-described novelOSCM 26, the combining function can be performed using conventionaldevices which are external to the integrated optical chip. For example,there are a number of known means for combining multiple wavelengthsinto a common path. These means have been used in WDM systems. Thesimplest means is a series of dichroic mirrors. Another approach is touse a fiber WDM. In general, the OSCM 26 of the present invention canemploy any method used in WDM systems for combining differentwavelengths.

The structure of optical split switch matrix (OSSM) 24 of FIG. 7 isshown generically in FIG. 12. The OSSM 24 directly feeds ultrashortpulses (e.g., 1.55 μm) from UPL 10 into any one or several of the WCCs.The control of the ultrashort pulses from the input port of the OSSM 24to any of the WCCs is accomplished using either an electro-optic or theacousto-optic method, as described hereinbelow.

FIG. 12 illustrates the use number of 1×3 optical directional couplers60 to distribute the input radiation to any or all of the output ports.The 1.55 μm wavelength pulses are fed into an optical waveguidefabricated by TI or PE or TIPE on a ferroelectric substrate such aslithium niobate or lithium tantalate. All the waveguides have the samewidth cross-sectional area which is designed for single-mode propagationat the source wavelength. The condition of the splitting action isgoverned by applying voltages V1 or V2 to the 1×3 optical directionalcouplers 60. Appropriate mode converters 16 can used in WCCs to ensureoptimized device operations, i.e., minimum excess loss and highinteraction efficiency in the WCCs (see FIG. 2). The switching voltageapplied to the 1×3 OSSM can be synchronized to the switching of the OCSM26 described above. The OSSM 24 shown in FIG. 12 can be realized usingelectro-acoustic or electro-optical active switches. It is also possibleto use guided-wave optical gratings to realize the above OSSM.

FIG. 13 illustrates a novel implementation of a 1×3 OSSM based onsurface acoustic waves (SAWs) generated by the interdigital transducersIDT1 70 and IDT2 72. As shown in FIG. 13, IDT1 70 and IDT2 72 aredisposed on the substrate surface. The optical waveguide regions markedwith Δn1, Δn2, and Δn3 are of slightly higher index than the base 1×3optical waveguide structure. The substrate material as preferablyferroelectric with the waveguides being fabricated using TI. Theslightly higher index waveguide regions are fabricated using PE. Withproper annealing, the refractive index change can be minimized, asrequired by this configuration. When no electrical signals are appliedto the interdigital electrode transducers, the path of the 1.55 μm laserlight will propagate straight into the middle output port of the 1×3OSSM. When a voltage V1 is applied to IDT1 70, the generated SAWs willdeflect the 1.55 μm laser light into the first (e.g., upper) output portof the 1×3 structure. In the same manner, if a voltage V2 is applied tothe IDT2 72, the 1.55 μm ultrashort pulse is deflected into the third(e.g., lower) output port of the 1×3 waveguide structure. The directionand amount of deflection of the input pulse depends both on the appliedvoltages and the Δn values. Placement of the IDTs on the surface of thesubstrate can be optimized to improve efficiency. Efficiencies greaterthan 90% can be realized with such a configuration. The insertion lossis minimized by the presence of the Δn structure.

For the equal distribution of the input laser radiation into all threeoutput ports of the 1×3 OSSM, the Δn's of the three hybrid waveguideregions can be increased by shorter anneal times or longer PE times. Inthis mode of operation, both of the applied voltages V1 and V2 to theOSSM are required to optimize the equal splitting action.

Instead of using acousto-optic element for deflection of the inputpulses, the switching action can be implemented using an electro-opticinduced grating (EOG) by using a pair of grating metallic electrodes onthe ferroelectric substrates. FIG. 14 shows a novel implementation ofsuch a 1×3 OSSM. The 1×3 optical waveguide device and the presence ofthe three appropriate higher Δn regions are similar to those describedin the acousto-optic based 1×3 OSSM shown in FIG. 13. By applying avoltage V1 to the EOG1 80, a periodic refractive index change is inducedsimilar to that generated by an IDT number in FIG. 13. The period of themetallic electro-optic induced grating structure is designed such thatthe 1.55 μm input pulse is switched to the first (e.g., upper) outputport of the 1×3 waveguide device. If no voltage is applied, the 1.55 μminput pulse will go directly into the middle port of the 1×3 OSSMdevice. If a voltage V2 is applied to the EOG2 82, the incoming 1.55 μminput pulse is switched appropriately to the third (e.g., lower) outputport of the 1×3 device. The refractive index changes of the three hybridwaveguide structures in the 1×3 OSSM can be increased to allow for equalsplitting of the incoming pump laser radiation into the three outputports. Both of the EOGs 80 and 82 are then used to optimize thesplitting. The acousto-optic and electro-optic devices described herecan be, for example, those used for switching in telecommunicationcircuits. Of course, other means of switching for integrated opticalcircuits for telecommunuication applications can also be used.

As with the OCSM described above, the OSSM can be extended from a 1×3 toa 1×10 structure. Again, the critical limitation is the size of theferroelectric wafer. The hybrid PE sections in the larger than 1×3 OSSMelement can be realized by multiple PE processes to compensate for ishigher splitting losses at larger angular splits, as required by theoverall OSSM, WCC and OCSM configuration. The OSSM 24, WCCs and OCSM 26are preferably formed on a single substrate.

The specific configuration for the multi-wavelength source of thepresent invention is very much dependent on the application. With amulti-wavelength source in a system, one has a system with much expandedcapability. In accordance with one preferred embodiment, themulti-wavelength source of the present invention is used as a source fora two-photon microscope. The purpose of the laser is to allow a largernumber of dyes which require different excitation wavelengths to be usedin one system. This expands the usefulness of such microscopes. Forexample, it may useful to have the laser quickly tuned from 780 nm, to700 nm and to 850 nm. Similarly, it may be useful to have the laserquickly tuned or simultaneously generate pulses at wavelengths of 680nm, 780 nm and 915 nm (see FIG. 6). Such a laser can be used for thedyes which are favorably excited by each of these wavelengths.

For example, dyes rhodamine, HT29 and HOE33342 are excited by two-photonexcitation at 780 nm. These are used for labeling hepatocytes, coloncarcinoma and nuclei, respectively. Dyes Fura-2, Indo-1, GreenFluorescent Protein and FITC are excited by two-photon excitation at 700nm. These are most useful for labeling structures and tracking calciumtransduction in dendritic neural structures. Three photon excitation atroughly 280 nm using 850 nm from the laser can be used to causeserotonin, tryptophan and NADH or NAD(P)H to autofluroresce. Serotoninis a key gauge of neural activity as the chief aminergic chemical in thebrain NADH and NAD(P)H are used to track activity used in identifying,for instance, skin melanoma.

Since a multi-wavelength source is very desirable for conventionalconfocal microscopy as well, the source of the present invention can beused for conventional rather than multiphoton excitation by using a dyewith absorption at the fundamental wavelength rather than one-half thewavelength. It may be desirable to, disperse the temporal pulse to alonger pulse so the peak power is not sufficient for two-photonexcitation. Therefore, in this case, the laser must be capable ofswitching to the common wavelengths used in confocal microscopy, such as482 nm and 514 nm of argon ion lasers, 632 nm of HeNe and 780 nm ofTi:sapphire.

It is also very desirable to increase the power of ultrafast sources.Higher powers are needed for many applications such as those related tomachining. It has been demonstrated that ultrafast fiber lasers can beamplified to one watt with erbium amplifiers. Presently, however, poweris limited to about 10 watts, which is too low for many machiningapplications. Recently, Yterbium fiber amplifiers have been demonstratedto yield 40 watts output. These amplifiers are more efficient thanerbium and are preferred for higher powers. These fibers have a largebandwidth and can support a very short pulse, but there is not acommercial ultrafast source at these wavelengths (i.e., 1.04-1.12 μm).With an OPG frequency converter, the output of a more conventional(e.g., Erbium) ultrafast source can be converted to the wavelengthsupstream of the Yterbium amplifier. This allows for a very high powersource.

Another high power amplifier which can reach even higher powers isYterbium YAG. Yterbium YAG has been shown to give 200 watts of averagepower and again there is not a conventional ultrafast source at thewavelength of Yterbium YAG.

One of the main applications for the Yterbium amplifier is as an RGBsource for commercial display or printing purposes. Again, afteramplification in the Yterbium amplifier, an OPG waveguide device can beadded which can simultaneously or separately convert the ultrafastpulses to red, green, blue wavelengths. The integrated optics circuit ofthe type described above could also include this switching circuit toturn the colors on or off for the image formation. The ultrafast pulseshave the advantage in that the efficient conversion obtained with highpeak-power and large bandwidth at each color minimizes the speckleobtained from the laser (speckle makes the image appear grainy to theeye).

Optical coherence tomography (OCT) is being developed as a medical andophthalmic imaging tool. It is capable of using light to image throughhuman tissue which scatters light strongly. OCT has been demonstrated togive images with better resolution than other medical imaging techniquessuch as MRI, computerized tomography, or ultrasound. Axial resolution is10 microns, and can be reduced to 2 microns when using a short coherencelength light source such as a femtosecond laser. However, the depth ofimaging is limited to about 3 mm. One desirable feature of OCT is thatit can use a simple and cheap light source such as a superluminscentlaser diode. However, better performance is obtained using a mode-lockedlaser. For example, in in vivo imaging of the heart of a frog embryo, ittakes 20 seconds to acquire an image when using a superluminescentdiode, but only 0.25 seconds when using a mode-locked laser, allowingresearchers to capture the motion of the beating heart during diastolicand systolic phases. Rapid scanning (2000 Hz) can be employed to achievethis fast image acquisition. Both mode-locked Ti:sapphire andmode-locked Cr:Forsterite have been used for OCT. Cr:Forsterite isespecially well suited for imaging in biological tissues because of itswavelength (1300 nm); scattering effects that limit imaging depth arereduced at longer wavelengths. Because the method is compatible withfiber technology, it has been successfully used for endoscopy. Asreported by Tearney et al. in "Rapid acquisition of in vivo biologicalimages by use of optical coherence tomography"; Optics Letters, Vol. 21,No. 17; September 1995, incorporated herein by reference in itsentirety, a radially-scanning catheter-endoscope probe with rapid imageacquisition has been demonstrated. OCT has been demonstrated in a numberof clinical and research trials including: cancer detection in the humanstomach wall; subsurface imaging and histology of the porcine esophaguswall; performing optical biopsy to replace excisional biopsy; andmapping blood flow velocities using Color Doppler OCT (CDOCT). Coupledwith catheter, endoscopic, or laparoscopic delivery, OCT holds thepromise of enabling the screening and diagnosis of a wide range ofdiseases including cancerous and precancerous tissue changes without theneed for excisional biopsy and histological processing. In conjunctionwith conventional microscopy, OCT enables the imaging of internalstructures in living specimens without the need for sacrifice andhistology.

Therefore, for the purposes of OCT imaging in human tissue, a sourcelasing at 1.3 microns is desired. An erbium-doped fiber laser which isconverted with an OPG waveguide device to 1.3 μm would be suitable forthis application.

Having described preferred embodiments of a new and improved apparatusfor generating controllable multi-wavelength outputs from a singlemode-locked source, it is believed that other modifications, variationsand changes will be suggested to those skilled in the art in view of theteachings set forth herein. It is therefore to be understood that allsuch variations, modifications and changes are believed to fall withinthe scope of the present invention as defined by the appended claims.

What is claimed is:
 1. An ultrashort pulse generator for generatingultrashort optical pulses at a plurality of different wavelengths,comprising:an ultrashort optical pulse source generating ultrashortoptical pulses; a plurality of wavelength conversion channels forconverting a wavelength of said ultrashort optical pulses to saiddifferent wavelengths, each of said wavelength conversion channelscomprising an optical waveguide and including an optical parametricgeneration portion for parametrically generating said ultrashort opticalpulses at at least one of said different wavelengths.
 2. The ultrashortpulse generator according to claim 1, wherein the optical waveguide ineach of said wavelength conversion channels is formed in a substratecomprising a periodically-poled ferroelectric optical material.
 3. Theultrashort pulse generator according to claim 2, wherein saidperiodically-poled ferroelectric optical material is one of: lithiumniobate, lithium tantalate, and KTP.
 4. The ultrashort pulse generatoraccording to claim 1, wherein each of said wavelength conversionchannels converts the wavelength of said ultrashort optical pulses as afunction of at least one of: a temperature of the wavelength conversionchannel; a wavelength of light pumped into said wavelength conversionchannel; and a periodic-poling period of an electric field in saidwavelength conversion channel.
 5. The ultrashort pulse generatoraccording to claim 1, wherein said ultrashort optical pulse source is amode-locked laser.
 6. The ultrashort pulse generator according to claim5, wherein said mode-locked laser is an erbium-doped fiber laser.
 7. Theultrashort pulse generator according to claim 1, wherein said ultrashortoptical pulse source is a mode-locked Ti:sapphire laser or a mode-lockedCr:Forsterite laser.
 8. The ultrashort pulse generator according toclaim 1, wherein each of said wavelength conversion channels furthercomprises at least one harmonic generator for generating ultrashortoptical pulses whose wavelength is shorter than the wavelength of theultrashort optical pulses generated by said ultrashort optical pulsesource.
 9. The ultrashort pulse generator according to claim 1, furthercomprising an optical splitter for directing the energy of saidultrashort optical pulses from said ultrashort optical pulse source intoat least one of said wavelength conversion channels.
 10. The ultrashortpulse generator according to claim 9, wherein said optical splittercomprises a switching waveguide, having a first refractive index,connecting a single input optical waveguide to n optical waveguides ofsaid wavelength conversion channels, where n is an integer greater than1, said switching waveguide including n regions of different refractiveindex for guiding optical pulses from said single input opticalwaveguide to respective ones of said n optical waveguides.
 11. Theultrashort pulse generator according to claim 10, wherein said switchingwaveguide is formed in a ferroelectric optical material by titaniumindiffusion, and said n regions of different refractive index are formedby proton exchange.
 12. The ultrashort pulse generator according toclaim 10, wherein the energy of ultrashort optical pulses received fromsaid single input optical waveguide is distributed substantially equallyamong a plurality of said n optical waveguides.
 13. The ultrashort pulsegenerator according to claim 10, wherein said optical splitter furthercomprises a 1×n directional coupler for directing the energy ofultrashort optical pulses received from said single input opticalwaveguide into any single one or any combination of said n regions ofdifferent refractive index, whereby the energy of the ultrashort opticalpulses received from said single input optical waveguide is guided intoany single one or any combination of said n optical waveguides.
 14. Theultrashort pulse generator according to claim 13, wherein said 1×ndirectional coupler comprises n acousto-optic devices for generatingsurface acoustic waves capable of deflecting ultrashort optical pulsesreceived at said single input optical waveguide into respective ones ofsaid n optical waveguides.
 15. The ultrashort pulse generator accordingto claim 14, wherein said n acousto-optic devices are interdigitaltransducers.
 16. The ultrashort pulse generator according to claim 13,wherein said 1×n directional coupler comprises n electro-optic devicescapable of deflecting ultrashort optical pulses received at said singleinput optical waveguide into respective ones of said n opticalwaveguides.
 17. The ultrashort pulse generator according to claim 16,wherein said n electro-optic devices are electro-optic induced gratings.18. The ultrashort pulse generator according to claim 9, furthercomprising an optical combiner for directing ultrashort optical pulsesin each of said wavelength conversion channels into a single outputwaveguide.
 19. The ultrashort pulse generator according to claim 18,wherein said optical combiner comprises:an output waveguide; and noptical directional couplers, where n is an integer greater than 1, eachof which respectively couples one of n optical waveguides of saidwavelength conversion channels to said output waveguide, whereinapplication of a voltage to one of said n optical directional couplerscouples ultrashort optical pulses propagating in a corresponding one ofsaid n optical waveguides into said output waveguide.
 20. The ultrashortpulse generator according to claim 19, wherein ultrashort optical pulsesfrom a plurality of said n optical waveguides propagate in said outputwaveguide simultaneously.
 21. The ultrashort pulse generator accordingto claim 19, wherein said output waveguide comprises n axially extendingportions each having a different refractive index, wherein ultrashortoptical pulses coupled into said output waveguide from an opticalwaveguide i propagate substantially in axially extending portions 1through i, where i is an integer from 1 to n, such that ultrashortoptical pulses of different wavelengths propagate in said outputwaveguide substantially in a single mode.
 22. The ultrashort pulsegenerator according to claim 19, wherein said output waveguide is formedin a ferroelectric optical material by at least one of titaniumindiffusion and proton exchange.
 23. The ultrashort pulse generatoraccording to claim 1, further comprising an ultrashort-pulse amplifierupstream of said wavelength conversion channels for amplifying saidultrashort optical pulses.
 24. The ultrashort pulse generator accordingto claim 23, wherein said ultrashort-pulse amplifier is an erbium fiberamplifier.
 25. The ultrashort pulse generator according to claim 1,further comprising an ultrashort-pulse amplifier downstream of saidwavelength conversion channels, wherein said ultrashort-pulse amplifieris one of: a Ytterbium fiber amplifier and a Ytterbium YAG amplifier.26. An ultrashort pulse generator for generating ultrashort opticalpulses at a plurality of different wavelengths, comprising:a mode-lockedlaser generating ultrashort optical pulses; and a wavelength conversionsection including an optical parametric generation portion and includinga waveguide-based optical switching section connecting a single inputoptical waveguide to n output optical waveguides, where n is an integergreater than 1, including acousto-optical or electro-optical means foreffecting switching of optical pulses from said single input opticalwaveguide to respective ones of said n output optical waveguides. 27.The ultrashort pulse generator according to claim 26, wherein saidwavelength conversion section is formed in a periodically-poledferroelectric optical material.
 28. The ultrashort pulse generatoraccording to claim 27, wherein said wavelength conversion sectionconverts the wavelength of said ultrashort optical pulses as a functionof at least a periodic-poling period of said ferroelectric material. 29.A multi-wavelength ultrashort-pulse generator, comprising:a laser forgenerating ultrashort optical pulses at a single wavelength; a pluralityof wavelength conversion channels for converting said ultrashort opticalpulses to a plurality of respective different wavelengths; and anoptical switch for switching ultrashort optical pulses from any one orany combination of said wavelength conversion channels into a singleoutput channel.
 30. The multi-wavelength ultrashort-pulse generatoraccording to claim 29, wherein each of said wavelength conversionchannels includes an optical parametric generation portion whichparametrically generates the ultrashort optical pulses.
 31. Anultrashort pulse generator for generating ultrashort optical pulses at aplurality of different wavelengths, comprising:an ultrashort opticalpulse source generating ultrashort optical pulses; an optical pump forgenerating pump pulses at a pump wavelength; an optical waveguide forconverting a wavelength of said ultrashort optical pulses to saiddifferent wavelengths, said optical waveguide including: a firstharmonic generation section responsive to said ultrashort opticalpulses, for generating harmonic ultrashort optical pulses at a harmonicwavelength; an optical parametric generation section responsive to theharmonic ultrashort optical pulses and the pump pulses, forparametrically generating signal ultrashort optical pulses at a signalwavelength and idler ultrashort optical pulses at an idler wavelength; asecond harmonic generation section responsive to the signal ultrashortoptical pulses, for generating first output ultrashort optical pulses ata first wavelength; and a third harmonic generation section responsiveto said idler ultrashort optical pulses, for generating second outputultrashort optical pulses at a second wavelength, said optical waveguidetransmitting third output ultrashort optical pulses at said pumpwavelength; and an ultrashort-pulse amplifier downstream of said opticalwaveguide, for amplifying ultrashort optical pulses at a wavelengthdifferent from a wavelength of said ultrashort optical pulses generatedby said ultrashort optical pulse source.
 32. The ultrashort pulsegenerator according to claim 31, wherein said ultrashort-pulse amplifieris one of: a Ytterbium fiber amplifier and a Ytterbium YAG amplifier.33. An ultrashort pulse generator for generating ultrashort opticalpulses at a plurality of different wavelengths, comprising:an ultrashortoptical pulse source generating ultrashort optical pulses; an opticalpump for generating pump pulses at a pump wavelength; and an opticalwaveguide for converting a wavelength of said ultrashort optical pulsesto said different wavelengths, said optical waveguide including: a firstharmonic generation section responsive to said ultrashort opticalpulses, for generating harmonic ultrashort optical pulses at a harmonicwavelength; an optical parametric generation section responsive to theharmonic ultrashort optical pulses and the pump pulses, forparametrically generating signal ultrashort optical pulses at a signalwavelength and idler ultrashort optical pulses at an idler wavelength; asecond harmonic generation section responsive to the signal ultrashortoptical pulses, for generating first output ultrashort optical pulses ata first wavelength; and a third harmonic generation section responsiveto said idler ultrashort optical pulses, for generating second outputultrashort optical pulses at a second wavelength, said optical waveguidetransmitting third output ultrashort optical pulses at said pumpwavelength.
 34. The ultrashort pulse generator according to claim 33,wherein said first, second and third harmonic generation sections aresecond-harmonic generators.
 35. The ultrashort pulse generator accordingto claim 33, wherein said optical waveguide is formed in a substratecomprising a periodically-poled ferroelectric optical material.
 36. Theultrashort pulse generator according to claim 35, wherein saidperiodically-poled ferroelectric optical material is one of: lithiumniobate, lithium tantalate, and KTP.
 37. The ultrashort pulse generatoraccording to claim 33, wherein said ultrashort optical pulse source is amode-locked laser.
 38. The ultrashort pulse generator according to claim37, wherein said mode-locked laser is an erbium-doped fiber laser. 39.The ultrashort pulse generator according to claim 33, further comprisingan ultrashort-pulse amplifier upstream of said optical waveguide foramplifying said ultrashort optical pulses.
 40. The ultrashort pulsegenerator according to claim 39, wherein said ultrashort-pulse amplifieris an erbium fiber amplifier.
 41. The ultrashort pulse generatoraccording to claim 33, wherein each of said first, second and pumpwavelengths corresponds to one of red, green and blue light.